This proposal seeks support for a program of investigation to determine whether the two major metabolites of testosterone formed in the rat brain normally contribute to the activation of masculine sexual behavior by differentially affecting neurotransmission in neurons which utilize 5-hydroxytryptamine (5-HT) or dopamine (DA) as neurotransmitters. Earlier research suggested that estradiol and 5 alpha-dihydrotestosterone (DHT), which are formed in rat forebrain from testosterone, act synergistically to activate masculine sexual behavior whereas DHT inhibits the facilitation of feminine sexual behavior by estradiol. The proposed studies will: (1) Compare the neural loci at which implantation of DHT or estradiol most effectively facilitates mounting behavior in castrated rats treated systemically with the other steroid, and identify the neural sites where DHT most effectively inhibits estrogen-induced feminine sexual behavior. (2) Attempt to relate the behavioral effects of estradiol and DHT to changes in the functional activity of 5-HT and DA neurons by studying the action of these steroids in gonadectomized rats treated with appropriate diets, neurotoxins, and monoaminergic agonists and antagonists. (3) Assess the effects of testosterone and its metabolites on the metabolism of 5-HT and DA in brain regions known to bind these steroids, and (4) Begin to explore the possibility that testosterone and its metabolites affect postsynaptic events associated with neurotransmission at 5-HT and DA synapses.